1. Field of the Invention
The inventions described below relate generally to medical systems, medical devices and methods and more specifically relate to illumination of a work area such as a surgical field.
2. Background of the Invention
Illumination of body cavities or surgical fields for diagnosis and/or therapy has been limited by overhead illumination. High intensity incandescent lighting has been developed and has received limited acceptance along with semiconductor and laser lighting. These light sources have a heat and weight penalty associated with their use. Additionally, such lighting sources can be cumbersome due to creation of shadows in the illuminated body cavity or surgical field. Excessive heat can cause unwanted coagulation of blood, as well as unnecessary heating of a patient's body. Additionally, heat buildup can cause various components fabricated from some polymers to exceed their glass transition temperature and deform. Heat buildup may also cause optical properties of various components to be compromised. Some of these systems are heavy and the weight of these illumination systems makes them uncomfortable for an operator, especially during a lengthy procedure. Conventional light sources rely on fiber optic and similar waveguide materials to conduct light to a body cavity or surgical field. These conventional sources and materials often suffer from poor light transmission and conduction inefficiencies, which may exacerbate excessive heating problems and result in weak illumination of the body cavity or surgical field.
Examples of conventional waveguide polymers that have traditionally been used with some success in surgical illumination systems include acrylics such as polymethylmethacrylate (PMMA) and polycarbonates (PC) such as Lexan®. Effective illumination during surgery requires efficiently conducting light through these waveguide materials. During typical use waveguides constructed from these materials come into contact with various materials such as blood, water, fat, skin, and hardware from adjacent medical devices. Contact between the waveguide and these various materials (environmental media) can induce light transmission losses via frustrated total internal reflection (TIR). Light losses via frustrated TIR may also occur when such waveguides are attached/glued to other devices for mechanical or therapeutic reasons. Normally these materials conduct light via TIR, wherein light traveling within the material is completely reflected at a boundary interface of the material when the light strikes that boundary interface at an angle equal to or below a critical angle. During TIR a portion of electric and magnetic (E/M) fields that make up light will extend a short distance past the material's boundary interface (into the external environment) this happens because electromagnetic fields must be continuous. This portion of the E/M field that extends past the waveguide's reflective boundary is known as an evanescent wave. If the evanescent wave finds an absorptive media, or a media with a higher index of refraction, within a few wavelengths of the boundary, then the evanescent wave may couple into environmental media. This causes frustrated TIR wherein light that would normally be contained within the waveguide material leaks out despite TIR conditions being met. Frustrated TIR is more likely to happen for light striking the waveguide material's boundary interface at an angle close to the critical angle for TIR. Frustrated TIR causes significant losses in light conduction and transmission efficiency and exacerbates extraneous heating of the waveguide materials and surrounding tissues, and degrades the quality of illumination provided to the body cavity or surgical field. Applying reflective coatings in attempt to prevent light from leaking out of the waveguide can present problems as well. Absorptive losses within reflective coating materials can generate unwanted amounts of heat and reduce the optical transmission efficiency of the waveguide. Therefore, it would be advantageous to provide improved illumination systems having waveguides and materials that minimize light losses from frustrated TIR. At least some of these objectives will be met by the exemplary embodiments described below.